What are some of things I need to keep in mind when shooting inside an aquarium? By aquarium, I mean the kind into which people walk in.
I have had the following problems while shooting in such conditions.

Without a flash, there is a horrible blue cast.

With flash, everything gets blown out in the foreground and the background is dark.

I did fiddle with the white balance and managed to get half-decent pics.

Higher ISO gave really noisy pictures.

I would like to know if there is anything else I could do to improve the pictures.

4 Answers
4

Apart from the effect on the fish, which is an issue which needs to be checked with competent authorities and the aquarium operator or owner, you van often get good results with flash when photographing though glass using the following method. This is also useful for display windows, and glass cases.
I can usually eliminate the apparent existence of glass in many cases using this method.

Approach glass relatively closely. Maybe one foot or less.

Align the camera's axis at a significant angle to the glass so that reflections from the flash are reflected completely out of the scene. II usually angle it horizontally but angled from above or below also works. This of course limits the angle of shots you can take, but a reflection free angle shot is often superior to a more frontal shot with many reflections.

Set the camera so that the non flash illumination from reflections will be low compared to the flash illumination level. eg set camera to f8 or small aperture

Experiment with angle for best effect.

Non flash:

Blcoking the reflections at source works well if you can do it. Cocconing the camera against the glass in a dark refelction free zone can work wonders - and/or get you into trouble :-).

Work out where major reflection sources are coming from and try to block the light before it reflects. May not be practical depending on environment.

Position camera close to glass at an angle and place a hood over the camera and the glass around the camera. The more extensive the hood the better. This may get odd looks and/or rude comment - but also may get good photos. Will not be viable or allowable in all cases.

When all else fails you can get some interesting shots with very strong reflections in them. Find or make a dark or darker spot in the reflection field and position yourself so that the subject is in this area. The reflections then become a part of the image - sometimes to great effect. Sometimes not :-)

You will want to shoot without a flash, otherwise, as you say it will be a mess of reflections and dark areas.

I think what you need to do is set a custom white balance.

I assume, by "walk in" you mean where you are inside an under-water tube?
Take a piece of white paper in with you, and you can use it as a custom white balance reference card. Your camera instruction manual will explain how to do this in detail.

Last I was in an aquarium I regretted I only had my point and shoot camera, as I would have wanted to use Raw and manual settings.

I went with P mode and no flash, and I also pressed the lens against the window to avoid reflections. It is also a good trick to hold your hand around the lens and the window you are shooting through. Otherwise, you can save good images by retouching them.

Another thing , which is true in all hard conditions: take A LOT of photos. Most of them will look blurry and not where you wanted them to be.

Those creatures move fast sometimes and you need to start shooting at 3-6 fps before they come into the position you want. And due to the slower than desired shutter speed, you can also follow the fish you want to catch at the same speed, like manual dynamic image stabilization.

I would also use polarization filter, if the brightness allows it. Hoya HD CIR POL takes less light away (80% transmission vs 60% on most other filters).

Examples from my trip with my point and shoot camera:

Retouched reflections:

Follow focus for lightning fast subject (it just dived into the water):